This invention relates to improvements in electrically heated nozzles, including torpedos, spreaders and heater bodies, for plastic molding or die casting equipment, and to the method for making the same. The invention also relates to nozzle systems utilizing such electrically heated nozzles, and to a lead arrangement for such nozzles and a method for making such a lead arrangement.
Such nozzle devices may be inserted into the orifice of the sprue fitting of the equipment to maintain the temperature of the material flowing therethrough during injection into a mold or die. The nozzle is an integral unit and includes an internal heater body comprised of an insulated heater coil, which may have at its ends frangible ceramic spacer discs for holding the heater body axially spaced from the walls of its metal casing. The resistance wire is connected to leads that extend out of the nozzle for connection with a source of electric current, as the leads may exit through holes in the spacer discs or through an opening formed along the length of the nozzle.
The space inside the casing, not occupied by heater coil and leads, is filled and packed with insulation material, such as magnesium oxide powder or other similar heat transfer or ceramic material, like aluminum oxide or boron nitride, which occupies all voids within the casing, to maintain high heat transfer between the heater coil and casing. The casing may be shaped to fit into and form a part of the delivery channel for conveying molten material into a forming cavity.
One embodiment of the invention utilizes flexible green insulator strips, which may consist of mica flakes or ceramic particles impregnated in heat dissipatable material, and electrical resistance wire may be wound on these green insulator strips. The wire wound green insulator strips may be inserted into the casing whereupon the casing is packed with insulation, filling all voids between the casing and wire. The assembly is heated to bake out the heat dissipatable material and bind the insulating particles together, and the assembly is then compressed and the casing may be formed as desired. By selectively using a combination of the green strips of heat dissipatable material in conjunction with insulation powder, fabricated as described, the nozzle may be made in a shorter time with varried heat insulation and transmission characteristics but good electrical insulation characteristics, as to direct heat to the molding orifice and away from other parts of the nozzle.
The nozzle assembly should be compressed to eliminate air voids, usually by swaging or rolling, which compacts the insulation material firmly between the heater body and casing to insure that all voids in the casing are filled. The heated nozzle assembly is formed as an integral unit, thus affording optimum heat transfer characteristics between the heating core and casing. The open end of the nozzle casing must be sealed, preferably with fiber or nylon discs, to prevent slippage or loss of heater components during assembly and swaging of the device.
Thermocouple means may also be provided within the casing and connected to the heater wire internally of the casing or at the casing exit, and the assembly may be integrally formed with the thermocouple.
Because of the integral nature of a heated nozzle made in accordance with this invention, which permits nozzle fabrication of minimum cross section with high heat transmission and control such a heated nozzle is ideal for hot runner or runnerless molding, suitable for ring gating, valve gating, sprue gating, or edge gating systems.